It is known to provide data loggers for transport refrigeration units, such as transport refrigeration units for straight trucks, tractor-trailer units, containers, and the like, which are time based. In other words, predetermined variables of the transport refrigeration unit are monitored by temperature sensors, humidity sensors, pressure sensors, and the like, and the readings of the sensors are recorded synchronously, ie., all readings are stored at once at predetermined constant time intervals. The stored readings are down loaded to a printer or computer at the end of each trip, to provide a record for each load carried by the associated transport refrigeration unit during each trip thereof. A Version 2.1 data logger available commercially from the assignee of the present invention is of this time based type.
U.S. Patent application Ser. No. 08/293 616, filed Aug. 22, 1994, U.S. Pat. No. 5,437,163, which is assigned to the same assignee as the present application discloses a data logger for transport refrigeration units which, in addition to recording variables at predetermined time intervals, also records data asynchronously relative to predetermined events which occur randomly or at non-scheduled times, such as alarms generated as a result of a detection of a malfunction in the operation of the transport refrigeration unit. The data logger of the patent application mentioned above takes advantage of a distributed control arrangement for transport refrigeration in which a global data base is shared by a plurality of communication nodes tied together with a serial bus.
It would be desirable, and it is an object of the present invention, to convert a time based data logger to a time and event based data logger, by utilizing the basic structure of the time based data logger and associated transport refrigeration unit apparatus, to make such a conversion economically and commercially feasible. In other words, such a conversion should not require the addition of a serial communications bus, serial communications controller, and a shared data base, as utilized in the hereinbefore mentioned co-pending patent application.